Malian troops bolstered security at army checkpoints and villagers detained two youths allegedly strapped with explosives on Saturday after Islamists claimed responsibility for the country's first suicide attack.
Residents of a village near Gao, the largest city in the north, detained two youths they said were wearing explosive-rigged belts on the same road where the suicide bombing on Friday wounded a soldier at a checkpoint.
"We arrested two young men early this morning. They had explosive belts and they were riding on two donkeys," Oumar Maiga, the son of the local village chief, told AFP.
He said the pair, an Arab and a Tuareg, were detained some 20 kilometres (12 miles) north of Gao, where troops were fortifying checkpoints with sandbags and heavy machine guns and patrolling the city in heavy rotation in camouflage pick-ups.
Northern Mali is being torn by rising tensions between light-skinned Arabs and Tuaregs -- often accused of supporting the Islamist occupiers -- and their black neighbours.
In Timbuktu, a grave was discovered Friday containing several bodies including those of three Arab shopkeepers who had recently been arrested by the Malian army, the independent Mauritanian news site ANI reported.
Rights groups have accused the Malian army of summary executions of Tuareg and Arabs and called on the government to protect them from reprisal attacks.
In Friday's suicide blast, a young Tuareg dressed as a paramilitary officer rode a motorcycle up to a checkpoint and detonated an explosive belt, an officer said.
The Movement for Oneness and Jihad in West Africa (MUJAO) claimed the attack and vowed to carry out more against "the Malian soldiers who chose the side of the miscreants, the enemies of Islam".
MUJAO is one of a trio of Islamist groups that occupied northern Mali for 10 months before France sent in fighter jets, attack helicopters and 4,000 troops to drive them out.
The French-led operation, launched on January 11 as the insurgents advanced toward the capital Bamako, has succeeded in forcing the Islamists from the towns under their control.
But they are thought to retain a presence in the vast desert spaces of the country's north, and France is now anxious to hand over the operation to UN peacekeepers amid fears of a prolonged insurgency.
Two Malian soldiers and four civilians have already been killed by landmines, and French troops are still fighting off what Paris called "residual jihadists" in reclaimed territory.
Some villages around Gao, 1,200 kilometres (750 miles) northeast of Bamako, continue to support the Islamists, French and Malian security sources say.
In Bamako, heavily armed Malian soldiers surrounded a base housing rival paratroopers where a firefight the day before killed two adolescents and wounded another 13 people.
The clash between the rival units highlighted the deep divisions in the Malian military.
The paratroopers are loyal to ex-president Amadou Toumani Toure, ousted in a March 2012 coup, and were protesting an order absorbing them into other units to be sent to the frontline.
-- 'State of advanced disrepair' --
The fighting overshadowed the arrival of 70 EU military trainers, the first of what is to be a 500-strong mission tasked with training the Malian army to secure the north.
French General Francois Lecointre, leading the mission, said the Malian army was in "a state of advanced disrepair".
The nation imploded last year after the coup, waged by soldiers who blamed the government for the army's humiliation by a rebellion among the Tuareg, a north African people who have long complained of being marginalised by Bamako.
A month later, paratroopers launched a failed counter-coup that left 20 people dead.
With Bamako in disarray, Al-Qaeda-linked fighters hijacked the Tuareg rebellion and took control of the north, imposing a brutal form of Islamic law.
France's intervention has won broad support in Mali, its former colony.
But in neighbouring Mauritania, the opposition condemned President Mohamed Ould Abdel Aziz's recent expression of support for the campaign, warning: "The spark of this war is bound to reach Mauritania."
In Paris, prosecutors brought terrorism charges Saturday against four men arrested last week as part of an investigation into a network to send jihadists to fight in Mali.
The men have links to Cedric Lobo, a Frenchman arrested in Niger last year on his way to Timbuktu to join Islamist groups in northern Mali, a judicial source said.